However, it is also known that effective operation
of biological treatment systems relies on highly active microorganisms
carrying out the process, and therefore on the ratio of
resistant/sensitive microorganisms to the contaminant being treated
(depending on both its chemical nature and concentration).
Microorganisms can degrade a wide variety of organic contaminants
and can adapt to many different inhospitable environments.
However, there are a variety of pollutants, both man made
(xenobiotic) and natural, that are not easy to degrade biologically,
even over long periods of time -these are referred to as ‘‘refractory
or recalcitrant’’.